The invention relates to a machine for the crosscutting of a web of material, and particularly a paper web, to be conveyed as a multi-ply wed, comprising a crosscutter, and in particular a rotary crosscutter, web guiding means preceding the crosscutter in the direction of web travel, and a feed section consisitng of a pair of rolls and disposed between the web guiding means and the crosscutter.
When a multi-ply web moves between a pair of rolls, there is some shifting of the individual plies relative to one another. The extent to which the various plies shift depends on the nature of the material, for example, its surface roughness, the thickness of the web, and other characteristics. When webs so conveyed are crosscut, sheet-size differences are encountered in the individual sheet packets. This is widely known as the stair-cut effect.
Various measures have been resorted to in order to minimize the shifting of the individual plies. In a prior-art machine of the type outlined above, the feed section comprises special rolls in which a displaceable air cushion is disposed between the hub of the roll and a resilient cylinder. Through meshing gearing, the hub of the roll is joined to the cylinder so as to permit displacement in the radial direction but not tangentially. Because of this special roll design, the rolls are capable of being flattened in the clearance between them. A machine of this type is disclosed in German Pat. No. 1,786,341.
In another prior-art machine, the feed section does not have rolls with resilient cylinders but instead has a pair of conveyor belts which are rearwardly supported on air cushions. A machine of this type is disclosed in German Pat. No. 1,611,768.
Although shifting is less pronounced with these two machines than it is with a machine having a pair of rolls with nonresilient cylinders, these machines fall short of meeting today's requirements.